Note that only the two polar regions of the intrafusal muscle contract; the equatorial region, where the nuclei are located, does not contract because it has little contractile protein. Nevertheless, when the polar regions contract, the equatorial region elongates and regains its sensitivity. Conversely, when a muscle relaxes (α motor neuron activity drops) and thus elongates (if its ends are being pulled), a concurrent decrease in γ motor neuron activity allows the intrafusal fibers to relax (and thus elongate) as well and thereby prevent the tension on the central portion of the intrafusal fiber from reaching a level at which firing of the afferent fibers is saturated. Thus the γ motor neuron system allows the muscle spindle to operate over a wide range of muscle lengths while retaining high sensitivity to small changes in length.
For voluntary movements, descending motor commands from the brain in fact typically activate α and γ motor neurons simultaneously, presumably to maintain spindle sensitivity as just described. This has two important functions. By maintaining the muscle spindle’s sensitivity as the muscle changes length, the spindle remains capable of sensing, and signaling to the CNS, any disturbances to the ongoing movement that cause an unexpected stretch of the muscle, and this in turn allows the CNS to initiate both reflex (see next section) and voluntary corrections. Second, if the spindle were to become unloaded during the movement, this would oppose the intended movement by decreasing the excitatory drive, via the group Ia reflex arc (see next section), to the α motor neurons driving the agonist muscles.
As mentioned earlier, there are two types of γ motor neurons: dynamic and static (see
Fig. 9.2).
This allows the CNS to have very precise control over the sensitivity of the muscle spindle. Dynamic γ motor axons end on nuclear bag1 fibers, and static γ motor axons synapse on nuclear chain and bag2 fibers. Thus when a dynamic γ motor neuron is activated, the response of the group Ia afferent fiber is enhanced, but the activity of the group II afferent fibers is unchanged; when a static γ motor neuron discharges, the responsiveness of the group II afferent fibers and the static responsiveness of the group Ia afferent fibers are increased. The effects of stimulating the static and dynamic fibers on a group Ia afferent fiber’s response to stretch are illustrated in
Fig. 9.5 . Descending pathways can preferentially influence dynamic or static γ motor neurons and thereby alter the nature of reflex activity in the spinal cord and also, presumably, the functioning of the muscle spindle during voluntary movements.
The Phasic (or Ia) Stretch Reflex
The reflex arc responsible for the phasic stretch reflex is depicted in
Fig. 9.6 ; the rectus femoris muscle serves as an example. A rapid stretch of the rectus femoris muscle strongly activates the group Ia fibers of the muscle spindles, which then convey this signal into the spinal cord. In the spinal cord, each group Ia afferent fiber branches many times to form excitatory synapses directly (monosynaptically) on virtually all α motor neurons that supply the same (also known as the homonymous) muscle and with many α motor neurons that innervate synergists, such as the vastus intermedius muscle in this case, which also acts to extend the leg at the knee. If the excitation is powerful enough, the motor neurons discharge and cause a contraction of the muscle. Note that the group Ia fibers do not contact the γ motor neurons, possibly to avoid a positive-feedback loop situation. This selective targeting of α motor neurons is exceptional in that most other reflex and descending pathways target both α and γ motor neurons.
Other branches of group Ia fibers end on a variety of interneurons; however, one type, the reciprocal Ia inhibitory interneuron (black cell in